This is an early reference to the familiar 'fishbowl' style of space helmet, fortunately constructed of pure glassite, a strong, clear material. Not invulnerable, however; a strong blow from a metal bar would shatter the material.

Kent and Liggett prepared to don their space-helmets, and before entering the airlock, Kent turned to Krell.

The debris among the pack's wrecks was just as varied, holding fragments of metal, dark meteors of differing size—and many human bodies. Among these were some clad in the insulated space-suits, with their transparent glassite helmets.

In space, it is of course important to have a secure seal for your space helmet. Hamilton solved this basic problem in the simplest possible way:

The airlock's inner door then slid open and the newcomers stepped into the ship's interior, unscrewing their transparent helmets as they did so.

We also have a nice illustration, from the cover of Astounding Science Fiction.

('transparent glassite helmets' from Sargasso of Space)

Manly Wade Wellman is usually credited with the creation of the phrase "space helmet" (see space helmet) in his novella The Disc-Men of Jupiter. However, since both stories were published in September of 1931, I'd call it a tie, wouldn't you?